The mouse embryoid body/teratocarcinoma system OTT 6050 developed by Leroy Stevens appears to be an ideal system for studying the molecular mechanism of cellular differentiation in a mammalian system. Total poly(A)-RNA was isolated from simple embryoid bodies and teratomas derived from the embryoid bodies. By RNA-complementary DNA hybridization analysis, an evaluation of the sequence complexity of the preparations could be made by computer analysis. Three classes of poly(A)-containing RNA could be resolved in both embryoid bodies and teratomas based on their relative abundance in the respective RNA preparations. By similar analysis nuclear poly(A+)-RNA was analyzed. Antibodies to brain specific proteins, S-100 and glial fibrillary acidic protein are being prepared to be used as probes for gene expression during neuroepithelial differentiation in the teratocarcinoma system.